Magnetic testing



April 4, 1939. T. c. HANA MAGNETIC TESTING Filed Jim@ 17, 1957 ATTO'RN EYS Patented Apr. 4, 1939 .UNITED STATE-s 2,152,690 MAGNETIC TESTING Thomas C. Hana, Flushing, N. Y., assigner to Magnetic Analysis Corporation, a corporation of New York Application June 17, 1937, Serial No. 148,645 claims. (c1. 1ra-lss)l This invention relates to electrical measurement and particularly to the determination oi' the wave form of alternating electrical quantities in magnetic analysis. It represents improvements 5 and modifications in' the method and apparatus described in co-pending applications Serial Nos. 136,824 and 136,825, both led April 14, 1937, by Theodor Zuschlag.

Modern methods and apparatus for magnetic l0 analysis are, at best, somewhat complicated and modifications which simplify commercial testing apparatus and increase reliability within the lim'- its of accuracy required represent marked advances in the art, even when the modification entails the sacrice of refinements which are only necessary in precision testing. In other words,

' in commercial magnetic testing it is desirable to simplify apparatus and procedure as long as the results are sufficient for commercial purposes.

In magnetic analysis involving the determination of the wave form of an alternating electrical potential experience has shown that the potential curve is usually symmetrical. In other words, the positive nodesl are similar to the negative nodes. Under such conditions. it may be suilicient to investigate alternate half cycles of the same sign with the apparatus discussed in the aforementioned prior application. However, I have found that it is also possible, and from a commercial standpoint desirable, to rectify the potential and investigate successivev cycles of the' resulting rectified potential. This change in procedure permits considerable simplification of testing apparatus.

The method of wave form determination `according to my invention comprises impressing a rectified signal voltage on a circuit, introducing into the circuit a bias voltage sufcient to prevent the flow of current past a point in said circuit,

40 reducing said bias voltage for a brief interval at corresponding instants in successive cycles of the rectiied signal voltage to permit a flow of current past the point in said circuit and determining the current flow past the point in said circuit when said bias voltage'is reduced.

My invention will be more clearly understood if reference is made to the following detailed description taken in conjunction with the accompanying drawing in which Fig. l is a circuit diagram of a presently preferred form of the apparatus of my invention;

Fig. 2 illustrates the combination of a rectied signal voltage and a bias voltage in the apparatus ofFig. 1;

Figs. 3a and 3b illustrate current impulses set up in the resistances 29, 29 of the apparatus of Fig. 1; and

Figs. 4a and 4b illustrate the dischargeof the condensers 30. 3l of the apparatus of Fig. 1.

Referring now to Fig. 1 it will be observed that 5 a primary winding I (preferably of low ohmic resistance) is connected in series with an ammeter 2 and with a variable current limiting resistor 3 to an alternating current supply line 4 such, for

example, as a cycle commercial power supply 10 connected to each other in series aiding. Their 20 outer ends are connected respectively to plates 1a, 1b, of a rectifying tube 1, while their common point is connected to a cathode 'Ic of this tube in series with a fixed resistance 8.

'I'he power supply line 4 is also connected to a 25 phase shifting transformer 9, the output vof which vis connected to the inputwinding I0 of a full wave rectifying transformer II in series with a condenser I 2 and a resistance I3. The input `circuit IIJ, I2, I3 is connected to the output of the 'phase shifting transformer 9 through a two-pole double-throw switch I4', which latter also permits a direct coupling of the input circuit I0, I2, I3, to the power supply line 4.

The full wave rectifying transformer II is pro- 35 videdwith a secondary output winding I5 the ends of which are connected respectively to plates Ilia, I6b of a full wave rectiiier tube I6. vThe midpoint of the output winding I5 is connected to cathode I6c of the full Wave rectifier tube 40 through a Xed resistor II. The end of the iixed resistor I'I adjacent the cathode I6c is grounded, for convenience, at G and is also connected to twin cathodes I8 of a detector tube I9. l 45 'I'he end of. the fixed resistor II remote from the cathodeA IGc is connected to one pole 2 la of a single-pole double-throw switch 2l, the contact arm of which is connected to the grid 22 of the detector tube I9 which is also equipped with a 50' similar grid 20.

The other pole 2 I b of the switch 2 I is connected with a slider 23 of a potentiometer 24. An external direct current source 25 is Vconnected to the two ends of thepotentiometer 24 to'provide for 65 the energization of the latter. The upper end of the potentiometer is connected to the negative pole of the power source, and thelower end of the potentiometer is connected to the positive pole of the power source. One end of the poten-v tiometer 24 (the lower end as shown in Fig. 1) is connected to the resistor 8 adjacent the cathode 1c. l

A voltmeter 46 is connected across the lower end of potentiometer 24 and the slider 23 of this potentiometer.

The amplifier detector tube I9 is provided with a pair of plates 26, 21, which are connected to-v gether through like resistors 28, 29, in series. A condenser 38 is shunted across the resistor 28 and a similar condenser 3| is shunted across the resistor 29. 'Ihe common point of the -two condensers is `connected to a twin cathode 35 of an amplier tube 36. Grids 31, 38, of this tube are connected respectively to the plates 26, 21, of the amplifier detector tube |9. The amplifier tube 36 isl also provided with a pair of plates 4I, 42, which are connected respectively to the ends of a potentiometer 39, which is provided with a slider 48. One end of the potentiometer 39 is connected to a slider 45 of another potentiometer 4'4, while the opposite end of the potentiometer 39 is connected to an end oi' the potentiometer 44. f

A sensitive direct current galvanometer 43 is connected across the ends of the potentiometer 44'.

An additional potentiometer 58- is connected at one end (the upper end in Fig. 1) to the slider 40 of the potentiometer '39, while the other end is connected to the ground at G. The potentiometer 58 is provided `with a slider 5| which is connected to the lead between the common points of the condensers 38,*3I and the twin cathodes 35 of the tube 36. A source of direct current 52 is connected across the potentiometer 50. (The-upper or positive pole ofthe source 52 is connected to the upper end of the potentiometer 58.)

Means (not shown) are provided for inserting I a magnetizable body A to be tested in the field of the primary winding I. The primary winding I and the two secondary windings and 6 are preferably disposed concentrically with each other so that the specimen may be inserted through them.

The principles of operation of the apparatus of Fig. 1 may be explained as follows:

If an alternating current be supplied to the primary winding I, alternating electromotive forces' will be induced in the secondary windings 5 and 6. 'I'he magnitude of the primary current supplied to the winding I may be controlled by the variable resistor 3 and read on the ammeter 2.

The alternating electromotive forces produced in coils 5 and 6 are imposed upon the plates 1a, 1b, of the tube 1 andrectified, thus producf ing a rectified signal voltage (shown as curve S on Fig. 2). If switch 2| is closed to the left, a resultant full Wave rectified signal voltage across resistor 8 is imposed between the left grid 22 and the twin cathode I8 in series with the potentiometer 24 and the resistor I1. In other words, the rectied signal voltage is impressed in series with the resistor I1 across the grid 22 of the de tector tube and the cathode I8. Thus a voltage is supplied to the grid 22 in such a manner that it tends to increase the current iiow to the plate 26 of the tube I9 through the resistor 28 and thereby produces a voltage difference between the grid 31 of the amplifier tube 36 and the cathode 35 of this tube. (It will be noted that the plate current is supplied to the plate 26 through the resistor 23 from the potentiometer 58, which in turn is energized by the direct current source 52.)

At the same time that the signal voltage from the coils 5 and 6 is being rectified and impressed between grid and cathode of the tube I9 (as just described) a full wave rectifiedbias voltage of like frequency is created and imposed across the resistance I1. 'I'he creation of this bias potential is simple. Current taken directly from the supplyline 4 or from the transformer 9 is imposed upon the primary of the transformer II. Electrical potentials-are thus set up in the two halves of the secondary I5 of the transformer.

II, and rectiiled by the rectifier I6 to produce a full wave rectified bias voltage shown as curve B on Fig. 2. y

The signal voltage to be analyzed is indicated as curve S on Fig. 2, while the curve T is the resultant of the rectified bias and signal voltage. The phase relationship between the bias voltage B and the signal voltage S may be varied by varying the setting of the phase shifting transformer 9, or if the potential is supplied directly from the line to the transformer there will be a iixed diiierence in phase between the signal voltage and-the bias voltage due to the condenser I2. In other words, the curve S may be shifted to the right or left with respect to curve B, or a iixed out o1' phase relationship may be maintained between them.

It will be apparent that the rectified bias voltage is applied to both grids 22, 28 of the detector tube I9 in parallel, and if the switch 2| is closed` to the right so that the signal voltage is not imposed on grid 22, like current flow conditions will be imposed upon both the plates 26, 21 of the detector tube I9. During most of the cycle the tube. I9 will be biased below cut-01T by the full wave rectified voltage, but when the bias voltage rises above cut-olf potential (as indicated at point U, for example, on Fig. 2) small and equal plate current impulses will occur at the same time in the resistances 28, 29. Such small current impulses are illustrated in Fig. l3a, and serve to charge the respective condensers 38, 3|.

During the portion of each cycle when the detector tube is not conductive (indicated by the V portion of the curve T onl Fig. 2) the con densers 38 and 3| will discharge as' shown in Fig. 4a. (The presenceoi the condensers in the circuit smooths the current impulses supplied to the galvanometer 43 and thereby improves the operation.) However, since the charges on both sides of the plate or output circuit of the detector tube are equal they balance each other and, if the amplier tube 36 is balanced, i. e. if the poh tential drops between plates 4| and 42 and slider 48 are identical, n'o voltage is imposed upon the galvanometer 43, and it will not be'deiected.

If nowthe switch 2| is closed to the left, Ithe rectiiied signal voltage is imposed on the grids 22 and 28 in series while the bias voltage is 'imposed on these grids in parallel. 'I'his creates an unbalanced condition, and the impulses at the plate 26 will no longer be like those at plate 21 of the detectortube. Consequently the currents through the resistances 28 and 29 willbe unequal and the condensers 38 and 3| will be unequally charged. Thus the current impulses supplied to one of the resistances 28, 29 will be to the current supplied to the other as the impulses indicated on Fig. 3a are to the impulses indicated on Fig. 3b. Likewise the discharge from one of the condensers 30, 3i will be to the discharge of the other condenser as the discharge indicated on Fig. 4a is to the discharge indicated on Fig. 4b. Under these circumstances there will be an indication of current at the galvanometer 43.

-In order to restore the galvanorneter 43 tov its original position the .effect of the signal voltage maybe balanced out with direct current potential from the source imposed upon the circuit in regulated magnitude -through the potentiometer 24.

The function of the tube 36 is lone of ampliiication, and for this purpose it receivesplate current through the potentiometer 50. The potentiometer 39 is employed for obtaining balanced conditions in the circuit; the potentiometer 44 serves to control the sensitivity and degree of deflection of the meter 43, and the potentiometer 50 serves to establish optimum plate currents in both the amplifier tube 36 and the detector tube I9. Bearing the above fundamentals in mind, the

operation of the apparatus may be explained asA tial set up in a conductor. disposed in inductive relationship with the body. 4Certain characteristics, such as hardness, may be indicated by achange in the magnitude -of the wave at some particular phase in each cycle. 'Ihe instant apparatus permits an investigation of the magnitude of the Wave at any phase in the cycle, and hence by a series of readings it is possible to reproduce the wave form. However, in most commercial work it is sufficient to investigatethe magnitude of the wave at one or two predetermined points with respect to phase. An investigation of this character is carried out by first establishing the phase point at which the wave of the signal voltage should be investigated, by establishing a phase difference between signal voltage and the bias voltage with the phase shifter 9er the condenser I2. 'I'hen with the switch 2| closed to the right to eliminate any effect of signal voltage upon the detector tube I9, the bias voltage (which should have an amplitude several times that of the signal voltage) is impressed upon the detector tubes. The circuit is then adjusted to get a maximum deflection of the galvanometer 43 with aminimum amount of potential by adjusting the slider 5I on the potentiometer 50. Then the galvanometer 43 is brought to a zero position by adjustment of the slider 40 of the potentiometer 39.

With a magnetic specimen of known and de.- sired magnetic characteristics in the field of the primary winding I, the switch 2i is next closed to the left. A deflection will probably occur at the galvanometer 43, but this deflection is wiped out by adjusting the slider 23 of the potentiometer 24 to introduce a. D. C. potential to equal and oppose the signal voltage impulse at the particular phase in question. This having been done, the stan-dard is replaced by a magnetic specimen of unknown characteristics. If the standard and the specimen are dissimilar with respect to a characteristic indicated in the particular phase of the signal potential being investigated, there Will be a deflection of galvanometer 43. If such a `defiection occurs, the specimen is not up to standard and should be rejected.

By varying the setting of the phase shifting transformer it is possible to change the phase relationship between the full wave rectified bias voltage imposed across the resistor Ii and the signl voltage imposed across coils 5 and 6.

If a complete picture ,of the signal voltage in any ,particular case is desired, the galvariometer 43 is` brought to zero by adjusting the potentiometer 23 and*` the reading of the voltmeter 46 is taken. This measures the amplitude of the sigreadings against the respective phase readings.

the wave of the duced.

'I'he duration of the time intervalin which the tube I9 is conductive is determined by the magnitude `of the full wave rectified bias voltage produced in the rectifier I6 and imposedacross resistor Il. The magnitude of this voltage can be adjusted, in case it is desirable or necessary,`by changing the value of the resistor I3. In general, the maximum amplitude of thev bias voltage should be 4 or 5 times that of the signal voltage.

Theintroduction of a D. C. potential at the potentiometer 24 to neutralize the effect of the signal voltage will be recognized as a null method. By the use of this null method nonlinearities of the tubes I9 and 3'6 do not interfere with the accuracy of the measurements, provided of course that the circuit was properly balanced and adjusted to start with. It must be borne in mind, however, the particular arrangement described above averages the individual amplitude values for two successive half cycles of the unrectied signal voltage and therefore does not produce a true picture of the signal voltage unless its successive unrectied half cycles were similar except as to sign. This fact, however, is of minor importance in most magnetic analysis testing problems, because generally it is not necessary to differentiate between the amplitude values of successive half cycles.

I claim:

1. In a method of magnetic analysis which involves producing an alternating signal voltage in a circuit and infiuencing said voltage by disposing a magnetizabie body to be analyzed in inductive relationship with the circuit. the improvement which comprises rectifying the signal voltage in the circuit, introducing a bias voltage in the circuit suflicient to prevent the ow of current past a point in said circuit, reducing said bias voltage for a brief interval at corresponding instants in successive cycles of the signal voltage rectified signal voltage is repro- `to permit a ow of current past said point, and

I bias voltage for a brief interval at corresponding instants in successive cycles of the rectied signal voltage and determining the current flow past said point in said circuit when said bias voltage is reduced.

ing brief corresponding intervals in successive cycles ofthe signal voltagel and means for measuring the output of the tubevduring said. intervals. 4. A method of determining an instantaneous value of a signal voltage in successive cycles thereof which comprises impressing the signal voltage on a circuit, rectifying said signal .voltage -in the circuit, introducing a bias voltage into the circuit suicient to prevent the ow of current past a point in said circuit, reducing said bias voltage for a brief interval at corresponding instants in successive cycles of the signal voltage to permit a flow of current past the point in said circuit and determining the current iiow pastithe f point in said circuit when said bias voltage is reduced. u i

5. In apparatus for determining the wave form vof electrical quantities including a vacuum tube detector having its input side connected to a Jrst circuit and provided with an output circuit,

means for impressing a signal voltage in the first circuit, means for impressing on said tube a bias voltage of varying magnitude such that the tube is rendered non-conductive except during brief corresponding intervals in successive cycles of a signalv voltage, and means for measuring the output ofthe tube during said intervals-'the improvement which comprises. means in said ilrst' circuit forvrectifying the signal voltage.

THOMAS C. HANA. 

